There is currently an industry push for wireless connectivity of field equipment in the process industry, which poses a number of additional challenges.
A process measurement instrument adapted for wireless communication generally includes a measurement unit, responsible for acquiring a measurement of a process variable, e.g. the level of a product in a tank, and a wireless communication unit, responsible for external communication. The instrument is typically driven by an internal power store, such as a battery, with a requirement on minimum lifetime often on the order of several years.
In order to conserve power in such battery-operated devices, and thus extend their lifetime, the measurement unit typically follows an independent duty cycle of activity and sleep mode. At the same time, the wireless communication unit typically operates intermittently, often as a result of complying with a wireless protocol, based on e.g. TDMA. This may result in random overlaps where either none, one or both units are active at the same time during operation.
As a result, the aggregated power consumption may present periods of extremely low power consumption, with both measurement unit and communication unit inactive, and periods of extremely high power consumption, with both measurement unit and communication unit active. Such steeply varying power consumption is not ideal for achieving a long battery life, and requires a very high maximal supply current.
An additional problem is present in instruments that rely on RF and/or microwave transmission for their measurement process. An example of such measurement instruments is a radar level gauge (RLG). In such instruments, there is a potential for mutual electromagnetic interference (EMI) between the RF module (or microwave module) and the wireless communication module when they operate concurrently. Interference can either be transferred within the enclosure/housing of the device or via the environment if the tank does not shield the radiation (e.g. plastic tanks).
Even if the modules operate at different frequencies, they usually have to be integrated in proximity of each other, in order to minimize the form factor e.g. reduce the board area or number of electronics boards. This can result in unreasonably high blocking specifications which require the use of rather expensive discrete RF filters which also degrade receiver performance by insertion loss and noise factor (they are usually arranged preceding any gain stage and have therefore a larger influence on the noise factor).
One option to solve the problem with mutual interference is to arrange for the modules to transmit/receive according to a common time division scheme, where only one module is active at a time. However, this would require some type of synchronous control of operation of both modules by a common control application. From an integration point of view this is not a desirable requirement, since the pace of development and origin of the two modules follow different design paths. For instance, radar level gauging modules are highly specialized measurement instruments, while wireless communication modules follow the rapid, but sometimes erratic, path of evolving standards.
Hence it is preferred to operate the modules each with its separate control application, following an operating scheme or protocol that is optimized separately. This calls for a different approach when trying to address the problems mentioned above.
In addition, the modules need some ways of communicating with each other, as it is the measurement result obtained by the measurement unit that is to be transmitted over the wireless channel. This implies at least some simultaneous operation, at least when transferring the current measurement value from the measurement unit to the wireless communication unit, e.g. a serial link.